1. Field of the Invention
This invention relates to the detection of small magnetized particles (beads) by a magnetic sensor, particularly when such particles or beads are attached to molecules whose presence or absence is to be determined in a chemical or biological assay.
2. Description of the Related Art
Magnetic devices have been proposed as effective sensors to detect the presence of specific chemical and biological molecules (the “target molecules”) when, for example, such target molecules are a part of a fluid mixture that includes other molecules whose detection is not necessarily of interest. The basic method underlying such magnetic detection of molecules first requires the attachment of small magnetic (or magnetizable) particles (sometimes denoted “beads”) to all the molecules in the mixture that contains the target molecules. Because of their small size these particles are “superparamagnetic”, meaning they ordinarily maintain no meaningful magnetic moment. However, when placed in an external magnetic field, these particles develop an induced magnetic moment and can produce a corresponding magnetic field.
The magnetic beads are made to attach to the molecules by coating the beads with a chemical or biological species that binds (e.g. by covalent bonding) to the molecules in the mixture. Then, a surface (i.e., a solid substrate) is provided on which there has been affixed receptor sites (specific molecules) to which only the target molecules will bond. After the mixture has been placed in contact with the surface so that the target molecules have bonded to it, the surface can be flushed in some manner to remove all unbonded molecules. Because the bonded target molecules are equipped with the attached magnetic beads, it is only necessary to detect the magnetic beads to be able, at the same time, to assess the number of captured target molecules. Thus, the magnetic beads are simply “flags,” which can be easily detected (and counted) once the target molecules have been captured by chemical bonding to the receptor sites on the surface. The issue, then, is to provide an effective method of detecting the small magnetic beads, since the detection of the beads is tantamount to detection of the target molecules.
One prior art method of detecting small magnetic beads affixed to molecules bonded to receptor sites is to position a magnetic sensor device beneath them; for example, to position it beneath the substrate surface on which the receptor sites have been placed.
FIG. 1 is a highly schematic diagram (typical of the prior art methodology) showing a magnetic bead (10) covered with receptor sites (20) that are specific to bonding with a target molecule (30) (shown shaded) which has already bonded to one of the sites. A substrate (40) is covered with receptor sites (50) that are also specific to the target molecule (30) and those sites should, in general, be different from the sites that bond the magnetic particle to the molecule. The target molecule (30) is shown bonded to one of the receptor sites (50) on the surface.
Referring to FIG. 2, there is shown a prior art magnetic sensor (60), similar to a structure used in magnetic random access memory (MRAM), that can be positioned beneath the receptor site of FIG. 1. As shown schematically in the cross-sectional view of FIG. 2, the prior art sensor (60) is a magnetic tunneling junction (MTJ), that includes a magnetized “free” layer (61) whose magnetization direction (610) is free to move and a magnetized “pinned” layer (63) whose magnetization (630) is fixed in direction. The two layers are separated by a thin, non-magnetic and electrically non-conducting layer (62), the tunneling barrier layer. The sensor is incorporated within a circuit that can detect changes in the magnetic direction of the free layer relative to the pinned layer, by sensing the changes in the resistance of the sensor, which change is a function of the change in the relative directions of the two directions. The circuit includes a selection transistor (70) having a source region (72) to which the sensor element (60) is electrically connected (65), a gate region (74) over which runs a conducting wordline (200) that can effectively activate the gate and allow a sensing current between the source (72) and a grounded (85) drain (76). An electrically conducting bitline (100) contacts the top surface of the sensor to external circuitry and can provide the sensing current that passes between source and drain, thereby effectively measuring the resistance of the sensor.
Referring to FIG. 3, there is shown a portion of a prior art MTJ array, formed by a regular repetition of the single circuit structure in FIG. 2. Four MTJ elements are shown (60a), (60b), (60c) and (60d) that are contacted by a common bitline (100) and accessed by four wordlines (200a), (200b), (200c) and (200d).
Referring to FIG. 4, there is shown an externally magnetized bead (10) positioned above an MTJ element (60), such the element of FIG. 2. Although the external field, H (arrows (400)) can be strong, it is directed perpendicularly to the plane of the magnetic moments (arrows (610) and (630)) of both the free and pinned layers of the sensor and, therefore, does not have an effect on their relative orientations. On the other hand, the field, H (400), induces a magnetic moment M (500) in the bead that, in turn, produces field lines, B (550) azimuthally surrounding the bead. The directions of B include components that are tangent to the plane of the free (61) layer of the element and that, therefore, can change the direction of the free layer magnetization (610). Such changes will produce detectable variations of the sensor's resistance and, in the case of an array of sensors and a large number of bound molecules with their magnetized beads, the overall resistance change of the array can be measured and the total number of captured molecules can be inferred.
Because the field, B, produced by the magnetized bead is fairly small, it is imperative to design MTJ sensors that have a high sensitivity. This is usually achieved by producing sensors with as low a magnetic anisotropy as possible, so that the magnetization direction is easily changed in direction. With such low anisotropy, however, the variations from one MTJ to another become significant and difficult to control. Therefore, it is difficult to design MTJ sensors that can reliably and consistently detect small magnetized beads.
Given the increasing interest in the identification of biological molecules it is to be expected that there is a significant amount of prior art directed at the use of magnetic sensors (and other magnetic sensors) to provide this identification. An early disclosure of the use of magnetic labels to detect target molecules is to be found in Baselt (U.S. Pat. No. 5,981,297). Baselt describes a system for binding target molecules to recognition agents that are themselves covalently bound to the surface of a magnetic field sensor. The target molecules, as well as non-target molecules, are covalently bound to magnetizable particles. The magnetizable particles are preferably superparamagnetic iron-oxide impregnated polymer beads and the sensor is a magnetoresistive material. The detector can indicate the presence or absence of a target molecule while molecules that do not bind to the recognition agents (non-target molecules) are removed from the system by the application of a magnetic field.
A particularly detailed discussion of the detection scheme of the method is provided by Tondra (U.S. Pat. No. 6,875,621). Tondra teaches a ferromagnetic thin-film based GMR magnetic field sensor for detecting the presence of selected molecular species. Tondra also teaches methods for enhancing the sensitivity of magnetic sensor arrays that include the use of bridge circuits and series connections of multiple sensor stripes. Tondra teaches the use of paramagnetic beads that have very little intrinsic magnetic field and are magnetized by an external source after the target molecules have been captured.
Prinz et al. (U.S. Pat. No. 6,844,202 and U.S. Pat. No. 6,764,861) teaches the use of a magnetic sensing element in which a planar layer of electrically conducting ferromagnetic material has an initial state in which the material has a circular magnetic moment. In other respects, the sensor of Prinz fulfills the basic steps of binding at its surface with target molecules that are part of a fluid test medium. Unlike the GMR devices disclosed by Tondra above, the sensor of Prinz changes its magnetic moment from circular to radial under the influence of the fringing fields produced by the magnetized particles on the bound target molecules.
U.S. Pat. No. 7,031,186 and Patent Application 2004/0120185 (Kang et al) disclose a biosensor comprising MTJ elements.
U.S. Patent Application 2007/0159175 (Prinz) shows on-chip magnetic sensors to detect different types of magnetic particles or molecules.
U.S. Patent Application 2007/0114180 (Ramanathan et al) teaches MTJ channel detectors for magnetic nanoparticles.
U.S. Patent Application 2005/0100930 (Wang et al) discloses detection of biological cells and molecules.
None of the prior art inventions provide a robust method of reliably detecting the presence of individual beads. It is the object of the present invention to provide such a method.